Field of the Invention
This invention relates to the bonding of aligned semiconductor wafers, and more particularly, to a semiconductor wafer bonder that bonds two, aligned, clamped wafers by applying (i) heat to the two wafers and (ii) a pneumatic force created by a pressure differential between a first pressure surrounding the two wafers and a second pressure, which is less than the first pressure, maintained between the two wafers.
Description of Related Art
Wafer-to-wafer bonding is used in a wide range of semiconductor fabrication applications for forming semiconductor devices. Wafer-to-wafer bonding involves the aligning of two wafer surfaces in contact with one another and then forming a bond interface between the two. The quality of the bond largely depends upon the preservation of the alignment of the two wafers and the uniformity of the heat and/or pressure applied during the bonding process.
Prior to bonding, the opposing surfaces of two wafers to be bonded are placed in contact with one another, aligned, and then held together in alignment by a wafer clamp. The wafer clamp typically includes first and second wafer clamping elements that contact the back surfaces of the two wafers, clamping the wafers together in alignment. During the bonding process, the wafer clamp, including the two wafers, is placed in a vacuum environment, which provides thermal insulation, helps prevent the wafers from oxidizing, and prevents molecular contaminants from collecting on the wafer surfaces. Mechanical elements, such as for example two hydraulic pistons, apply contact pressure directly onto the first and second wafer clamping elements respectively. In addition, heating elements, typically internal to the mechanical elements, apply heat via a direct mechanical thermal path onto to first and second wafer clamping elements respectively. In turn, the first and the second wafer clamping elements apply pressure and heat to the wafers, resulting in bonding between the two wafers.
The above-described wafer bonding devices have a number of problems. First, any shear forces created by the mechanical elements on the wafer clamping elements may cause misalignment of the wafers. Second, the vacuum environment creates a poor and non-determinant thermal coupling between the heating elements and wafers. As a result, non-uniform heat may be applied across the wafer surfaces. The net result of the possible misalignment and non-uniform heat is that a poor bond may be created between the two wafers. In addition, the integration of the heating elements within the mechanical elements means that neither the pressure nor the heating functions can be optimized.